1. Field of the Invention
The invention relates in general to a method of reducing the resistance of a contact (R.sub.c), and more particularly, to a method of reducing the resistance of a self-aligned contact (SAC).
2. Description of the Related Art
Due to the higher and higher integration of integrated circuits, the dimensions of semiconductor devices in the integrated circuits become smaller and smaller. The devices cannot provide a sufficiently large surface area to fabricating various interconnects for electrical connection between different electrodes in the devices, so cannot the integrated circuits provide a sufficient large surface area to fabricate various interconnects to electrically connect different electrodes or devices. As a consequence, two or more metal layers are required to achieve the interconnection between different electrodes or devices. Conventionally, these metal layers are insulated by various isolation structures or dielectric layers and coupled with each other by a contact via/plug penetrating through the dielectric layers.
Another consequence of high integration is that the line width of devices becomes shorter and shorter. While transferring a pattern of a contact window from a photo-mask, extremely high resolution and very little misalignment are required. Poor resolution can form a void within the contact window or form a short circuit or current leakage between devices due to a very large aspect ratio of the contact window. Any misalignment can cause a short circuit or a leakage current of devices after subsequent etching process. A self-aligned contact window is commonly used to help resolve these problems.
FIG. 1A to FIG. 1C show a conventional method of forming a self-aligned contact window. In FIG. 1A, gates 102 and spacers 104 around the gates 102 are formed on a substrate 100. A cap layer 106 is formed to cover the gates 102 between the spacers 104. The material of the spacers 104 and the cap layer 106 comprises silicon nitride formed by plasma chemical vapour deposition. The formation of the spacers 104 and the cap layer 106 prevents the gates 102 from being etched in the subsequent etching process. An oxide layer 108 is formed by conventional chemical vapour deposition.
In FIG. 1B, a photo-resist layer 110 with an opening 112 is formed on the oxide layer 108. The opening 112 is aligned over a predetermined source/drain region between the gates 102. Therefore, the oxide layer 108 on top of the predetermined source/drain region between the gates 102 is exposed within the opening 112. However, in case of a misalignment, the opening 112 exposes a part of the oxide layer 108 on top of the gates 102. An anisotropic dry etching is performed to etch the oxide layer 108 exposed by the opening 112, so that a contact window 116 is formed to expose the substrate 100 below the opening 112. Because of the high etching selectivity of oxide compared to the material of the spacers 104 and the cap layers 106, the exposed oxide layer 108 is removed without etching the spacers 104 and the cap layers 106. Even when the opening 112 is mis-aligned and the oxide layer 108 on the gates 102 are exposed, being protected by the spacers 104 and the cap layers 106, the gate 102 is also not etched or even damaged during the etching process that forms the contact 116 window.
The above process is called as a "self-aligned" method of fabricating a contact window. With the formation of spacers 104 around the gates 102 and the cap layers 106 on the gates 104, the gates 102 are protected from being etched or damaged. A contact window 116 is formed without causing leakage current or open circuit. However, as the line width of devices becomes narrower and narrower, the gates 102 become closer together and the contact surface of the contact windows 116 shrinks. The resistance of a conductive material, for example, tungsten, formed within the contact window in the subsequent process increases. One experiment shows that the resistance between the self-aligned contact and the substrate is as high as 111 to 113 .mu..OMEGA.cm. With such a high resistance, the contact cannot form a good electrical connection between different metal layers or electrodes. An open circuit thus occurs. There is thus a need for a way to improve the conductance of the contact window 116 as the line width becomes increasingly narrow.